Control of start-up and operation of anaerobic biofilm reactors: an overview of 15 years of research.

Anaerobic biofilm reactors have to be operated in a way that optimizes on one hand the start-up period by a quick growth of an active biofilm, on the other hand the regular operation by an active control of the biofilm to avoid diffusion limitations and clogging. This article is an overview of the research carried out at INRA-LBE for the last 15 years. The start-up of anaerobic biofilm reactors may be considerably shortened by applying a short inoculation period (i.e. contact between the inoculum and the support media). Then, the increase of the organic loading rate should be operated at a short hydraulic retention time and low hydrodynamic constraints in order to favor biofilm growth. After the start-up period, biofilm growth should be controlled to maintain a high specific activity and prevent clogging. This can be done in particulate biofilm systems by using hydrodynamics to increase or decrease shear forces and attrition but is much more difficult in anaerobic fixed bed reactors.

Competition between planktonic and fixed microorganisms during the start-up of methanogenic biofilm reactors.

The influence of the hydraulic retention time (HRT) on the start-up phase of a methanogenic inverse turbulent bed bioreactor was investigated. Two identical reactors were monitored, the only differing parameter being the HRT: one of the reactors was fed with a diluted wastewater at a constant HRT of 1 day, the organic loading rate (OLR) being increased by decreasing the substrate dilution; the second reactor was fed at a constant influent concentration of 20 g COD L(-1), the OLR being increased by decreasing the HRT from 40 days to 1 day. After 45 days of start-up, both reactors were operated at an OLR of 20 g COD L(-1)d(-1) and a HRT of 1 day. However, strong differences were observed on biofilm growth. In the reactor operated at a constant short HRT, biofilm concentration was 4.5 as high as in the reactor operated at an increasing HRT. This difference was attributed to the competition between planktonic and biofilm microorganisms in the latter reactor, whereas suspended biomass was quickly washed out in the former reactor because of the low HRT.

Influence of hydrodynamic conditions on the start-up of methanogenic inverse turbulent bed reactors.

The influence of hydrodynamic conditions on the start-up phase of an inverse turbulent bed bioreactor was investigated. Two identical reactors, differing only by the gas velocity ensuring the carrier fluidization and generating the main hydrodynamic strengths (attrition), were monitored. Regarding the first 96 days, the experiments showed better performances for the reactor having the lowest hydrodynamic strengths. These results were correlated to lower attached biomass and higher biofilm specific removal rate for the reactor subjected to strong hydrodynamic strengths. Once the start-up was completed and the reactor stabilized at an organic loading rate of 6g(COD)L(-1)d(-1), the same hydrodynamic strengths were applied (gas velocity was equalized). The results proved that hydrodynamic conditions make it possible to control the biofilm characteristics. Consequently, it should be recommended to apply minimal strengths to enhance the biofilm growth during the early start-up phase, and then to increase them to control biofilm thickness and to optimize the reactor performances.